The epithelium of the small intestine undergoes a remarkable series of morphogenic changes during its development. The endodermally derived epithelial tube is initially comprised of a single layer of short columnar cells. Between E10.5 and E14.5, the tube increases in girth and length. In the 48 hours between E14.5 and E16.5, the epithelium is dramatically remodeled and the previously flat luminal surface is converted into villi (fingerlike projections of the epithelial surface). Since villi represent the functional absorptive unit of the intestine, a detailed understanding of the mechanisms leading to their formation is an important goal. Moreover, failure to establish proper intestinal surface area (as in idiopathic short bowel syndrome) can be life threatening. Data presented here challenge current thinking about the organization of the early intestinal epithelium and the mechanisms by which the epithelium is remodeled at E14.5. The Working Model underlying this work is that: The early intestinal epithelium (E12.5-14.5) is pseudostratified and grows via symmetrical cell divisions called g-divisions (because they increase epithelial girth and length). At E14.5, luminal surface area is extended by means of a different type of specialized cell division, called e-divisions (because they serve to expand the luminal surface). In e-divisions, new apical surface is added at the cytokinetic furrow. E-divisions therefore separate daughter cells onto different villi. After e-divisions, postmitotic cells change shape, shortening along their apical/basal axis and increasing their apical surface, as the first villi emerge. This cell reshaping rapidly converts epithelial girth to length. The fidelity of these processes requires Wnt5a and planar cell polarity signaling. None of these aspects of intestinal growth have previously been investigated. This proposal makes use of genetic mouse models, a novel intestinal explant culture system and advanced imaging techniques to mechanistically dissect the interconnected signaling and patterning events involved in these surface-generating processes. The Specific Aims are designed to examine the morphological and functional characteristics of g-divisions (Aim 1) and e-divisions (Aim 2) and to investigate the mechanisms underlying intestinal lengthening, including the role of Wnt5a and planar cell polarity signaling (Aim 3). Using all of these data, a mathematical model will be generated to further investigate the role of such parameters as progenitor cycling time, apoptosis and cell shape change in intestinal lengthening. The outcome of these studies could have important implications for the bioengineering of fetal intestinal tissue and the treatment of short bowel syndrome in the fetus and newborn.